1. Field of the Invention
The present invention is concerned with explosives comprising a continuous phase of a first explosive having embedded therein discrete bodies of a second explosive. More particularly, the present invention is concerned with cast explosives of the type commonly referred to as booster explosives.
2. Related Art
Booster charges are solid explosive charges used to initiate blasting agents such as ammonium nitrate-fuel oil (ANFO) mixtures. Such booster charges are available in a variety of sizes and shapes, e.g., cylindrical, conical, etc., typically having weights from, e.g., 5 grams to 88 ounces, lengths of 4 to 30 inches and diameters of 0.5 to 5 inches. Booster charges may be composed of trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), cyclo-trimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), pentolite (a mixture of PETN and TNT), other types of explosives such as fuel-oxidizer mixtures, and various mixtures of these explosives. In addition, stabilizers, emulsifiers and other additives may be present in the explosive mixture of the booster charge. These explosives all have individual characteristics in terms of ease of initiation, explosive energy, brisance, shelf life, solidification point and other factors which impact safety and usage of the booster charges.
Booster charges are conventionally made by pouring into a container, which serves as a mold, a molten or otherwise pourable explosive material and solidifying it within the container. Solidification of the liquid explosive may be by means of cooling, polymerization, crystallization, chemical reaction, hydration, curing or other methods known in the art. The resulting charge may be of any suitable shape including cylindrical, conical, irregularly conical, spherical and polygonal. One cast booster charge representative of the prior art weighs about 12 ounces and may be about 4.7 inches long with a diameter 1.9 inches.
A suitable fixture may be placed within the container prior to pouring the pourable explosive therein to provide one or more initiator seats such as one or more bores (which may comprise passageways open at both ends or wells open at one end only) within the cast booster charge. An energetic initiation device or xe2x80x9cinitiatorxe2x80x9d, such as a low-energy detonating cord (LEDC) and/or a detonator, is placed within the initiator seat so that upon initiation of the initiator, the cast booster charge is detonated. Cast booster charges are conventionally used to detonate a larger mass of a blasting agent such as the well known ammonium nitrate-fuel oil mixture (xe2x80x9cANFOxe2x80x9d).
As used herein, the term xe2x80x9ccontact surfacexe2x80x9d or xe2x80x9cinitiation surfacexe2x80x9d refers to a surface on the booster charge, optionally at an initiator seat (e.g., a bore, passageway, well, groove, indentation, etc.) configured to receive an initiator, which receives the initiation signal from the initiator.
The art has been concerned with, among other things, preparing cast booster charges of sufficient sensitivity so that they may be reliably initiated by low-energy initiators such as low-energy detonating cord and relatively low-energy or small detonators. For example, in a typical environment of use, one or more cast booster charges are placed within a borehole which is partially filled with ANFO. The borehole may also contain some stemming material such as crushed gravel to seal the top of the borehole and/or to divide the borehole into two or more stages or xe2x80x9cdecksxe2x80x9d of ANFO. In any case, if the booster charges or detonators contained within the cast booster charges are to be initiated by detonating cord, the detonating cord must pass through the ANFO or other blasting agent. It is therefore desirable to use a low-energy detonating cord to avoid the possibility that detonation of the detonating cord will initiate the ANFO prematurely or alter its explosive properties prior to initiation of the cast booster charge.
FIGS. 1 and 1A (prior art) show a prior art expedient for increasing the sensitivity of a cast booster charge. To prepare charge 10, PETN 14a may be contained as a powder within a balloon which is wrapped around a straw 12a around which the main body (the continuous phase) of charge 10 is cast as an annular-shaped body 14b of a TNT-containing explosive such as pentolite or composition B (a mixture of RDX and TNT). In use, a low-energy detonating cord may be passed through passageway 16 as an initiator, and may be knotted below straw 12a in order to prevent its slipping out of passageway 16. The PETN therefore defines at least a portion of the initiation surface of charge 10. PETN is more sensitive than is the cast TNT-containing explosive, but it is also significantly more expensive. However, by providing PETN at the initiation surface, the reliability of initiation from the initiator is significantly improved. Upon initiation of the low-energy detonating cord (not shown) within passageway 16, the sensitive PETN 14a is detonated, which in turn detonates the less sensitive cast body 14b. Cast booster charge 10 is typically used to detonate a larger mass of a still less sensitive blasting agent such as ANFO, as is well known to those skilled in the art.
The prior art embodiment of FIGS. 1 and 1A has several drawbacks, including high production costs because of the necessity to fill balloons with the PETN and position and retain the balloon about the straw 12a and within a cylindrical container 12. Should the PETN balloon be omitted from one or more containers, the result would be a less sensitive, all TNT or TNT-based (or other explosive) cast booster charge which may not be sufficiently sensitive enough to be initiated by a low-energy detonating cord placed within passageway 16. The balloon may be misplaced, causing unreliable initiation. The invention eliminates this problem, and may incorporate the more sensitive explosives into a continuous phase to define the initiation surface. Optionally, the explosive material in the continuous phase may have a low permeability to water and so may not require isolation from water.
FIG. 1B is a cross-sectional view of another booster charge 600 according to the prior art. Cast booster charge 600 comprises TNT pellets 640, explosive filling 642 and a pentolite core 644. Pellet 640 and filling 642 are both composed of TNT only.
FIG. 1C is a cross-sectional view of still another prior art booster charge 700. Cast booster charge 700 comprises pentolite filling 750 and a less sensitive TNT-containing mixture for filling 752. There is no mixture of pentolite and TNT in this prior art charge. Note that if a detonator does not contact pentolite filling 750, sensitivity of the charge may not be sufficient to insure detonation.
It is further known in the art to make the booster explosive from a first explosive such as TNT and to contact or line the passageway with a second explosive which is more sensitive to initiation than the first explosive.
U.S. Pat. No. 4,776,276, issued to M. E. Yunan on Oct. 11, 1988 and entitled xe2x80x9cCast Explosive Primer Initiatable By Low-Energy Detonating Cordxe2x80x9d, discloses a cast booster charge which contains PETN disposed in a sleeve about the passageway through the charge where a detonating cord passes. The PETN about the passageway is more sensitive to initiation than the rest of the explosive material of the cast booster, so its close proximity to the detonating cord increases the reliability of initiation. Other prior art expedients include embedding a length of detonating cord at the passageway or providing a core of high PETN content surrounded by an annular body of a less sensitive explosive. The more sensitive, second explosive emplaced at the passageway is more reliably initiated by the detonating cord or detonator placed within the passageway and in turn initiates the remainder of the booster explosive.
U.S. Pat. No. 4,000,021, issued to Voigt, Jr. on Dec. 28, 1976 and entitled xe2x80x9cProcess For Suspending Particulate Additives In Molten TNTxe2x80x9d, discloses a process for suspending particulate additives in molten TNT. Composite explosive slurries are obtained by dispersing particulate solid components such as RDX in molten TNT in the presence of a water soluble gum, column 2, lines 10-16. The objective of the invention is to provide a process for dispersing particulate solids in molten TNT to allow production of cast explosive of uniform composition. Examples 1 and 4 reveal ammonium nitrate prills of particle size ranging from 150-1000 microns and examples 2-4 reveal use of RDX having an average particle size of 40 microns.
U.S. Pat. No. 2,384,730, issued to Davis et al on Sep. 11, 1945 and entitled xe2x80x9cMethod Of Preparing Cast Explosive Chargesxe2x80x9d, discloses a thorough mixture of wet particulate PETN with molten TNT. The PETN is preferably relatively finely divided (column 2, lines 9-12) and thoroughly mixed with the TNT. The practice of adding dried PETN to molten TNT, with the resultant formation of lumps (presumably of PETN), is noted (column 1, lines 1-9).
A company called Canadian Industries Limited or xe2x80x9cCILxe2x80x9d is believed to have manufactured a booster comprising a core of pentolite surrounded by prill and cast TNT on the outside.
It has been known in the manufacture of some military explosives to incorporate inert particulate material in order to increase the density of the explosive in the molten state. It is also known in the art to add solid particles of the molten material to control shrinkage and void formation in the cast body.
The prior art references do not disclose, either individually or in combination, an explosive comprising a plurality of larger discrete bodies (as opposed to powder particles (i.e., particles sized less than 1 mm)) of one explosive material or of an inert material, embedded within a continuous phase of another explosive material. These patents also do not disclose, individually or in combination, the mixture of discrete bodies of a less sensitive TNT-based mixture into a continuous phase of pentolite or the use of discrete bodies of materials comprising more than one explosive chemical compound.
The present invention provides an explosive charge comprising an explosive matrix material having therein a plurality of discrete bodies of a second material which is less sensitive to initiation than the matrix material.
In a particular embodiment, the matrix material may comprise a combination of PETN and TNT, and the second material may comprise TNT.
According to one aspect of the invention, the discrete bodies have a minimum dimension of at least 1 millimeter (mm), e.g., discrete bodies in the shape of pellets may have a diameter and length of at least 1 mm. In specific embodiments, the discrete bodies may be in the shape of round-ended cylinders having lengths and diameters of 0.8 centimeter (cm) or, optionally, 1.6 cm.
According to another aspect of the invention, the charge may define a contact surface for an initiator and the discrete bodies may be concentrated away from the contact surface to provide a region of high sensitivity near the contact surface.
According to still another aspect of the invention, the explosive charge may comprise a second plurality of discrete bodies of an explosive material.
The present invention also provides an explosive charge comprising an explosive matrix material having therein an interspersed phase comprising a plurality of discrete bodies of a second material, wherein the discrete bodies have a minimum dimension of at least 1 mm. Optionally, the second material may comprise an explosive material which is more sensitive to initiation than the material in the matrix material. For example, the matrix material may comprise TNT and wherein the second material may comprise pentolite. Alternatively, the discrete bodies may comprise an explosive material which is less sensitive to initiation than the matrix material, or they may comprise non-explosive material.